7 Questions About How The Immune System Works and What You Should Know About It

The circulatory system, the digestive system, the reproductive system: all of these body systems make sense. There’s an order to it all. There are distinct organs involved that we know the names of. We get it, no biology degree required (at least not for the basics). But then there’s the immune system, which may as well be a giant question mark floating in our bodies. While we vaguely know that the immune system’s job is to safeguard the body from bacteria, viruses and similar nasties that cause infection, illness and disease, the hows, whys and, heck, even the wheres are often a mystery.

What does it even mean to boost your immune system? What are we boosting? The truth is that “the immune system is remarkably complex, and the immune response is hard to get your head around,” says Suzanne L. Cassel, M.D., a clinical immunologist at Cedars-Sinai in Los Angeles. “Not only does the immune response affect every organ and every system in the body, but it’s also constantly changing and varies between people.”

But it is understandable. We promise. And once you get the essentials down, you’ll be better equipped to help your immune system work for you. The easiest way to put it is like this: “The immune system is your body’s own personal military. It’s in charge of defending you,” says Susan Blum, M.D., founder of the Blum Center for Health in Rye Brook, New York, and author of The Immune System Recovery Plan.

In order to protect and serve, your military has different divisions with various functions. It requires soldiers and weapons. “And just like any military, you need bases all over to ensure you’re fully protected,” says Blum. And when she says all over, she means all over.

1. Where Is the Immune System, Anyway?

It’s pretty much everywhere. Your immune system encompasses individual cells and proteins, organs and organ systems. In fact, the body’s largest organ—the skin—is a major part of the immune system and is considered the first line of defense against germs entering your body.

“It’s a physical barrier,” says Blum, who notes that other physical barriers include the corneas and membranes that line the respiratory and reproductive tracts. As long as these things are intact, invaders can’t enter the body. And to double down on protection, physical barriers also boast a backup team that includes sweat, tears, mucus and secretions that contain bacteria destroying enzymes.

And if a fungus, bacteria or a virus does happen to sneak in, “there are military bases scattered throughout the body, like in the lymph nodes, the bone marrow, the thymus and the spleen, where immune cells lie in wait, readying to be activated,” Blum says. These are all part of what’s called the lymphatic system, which is a network of tubes, tissues and organs that not only ushers out dead cells and germs but transports infection-fighting white blood cells all over.

Then there’s the gastrointestinal tract, which is like the ultimate hybrid defense. While the GI tract isn’t an official member of the lymphatic system, “it harbors almost 80% of the lymphoid tissue in the body and contains the majority of immune cells,” says Satya Dandekar, Ph.D., chair of the department of medical microbiology and immunology at UC Davis. And it’s another first line of defense.

When you think about it, the GI tract’s role in immunity makes total sense: we bring the outside world into our body all day long through our mouth. “The gut is the most important defense against foodborne illness, helping to make sure those pathogens don’t get absorbed into the blood,” says Julie LG Lanford, a registered dietitian nutritionist at Cancer Services, survivorship nonprofit in Winston-Salem, North Carolina. Plus, gut microbiota stimulates the immune system and acts as a kind of barrier to infection.

A 2018 report in the journal Frontiers and Immunology noted that healthy gut microbiota can essentially hinder a pathogen’s ability to stick around. It’s also impossible to have a healthy immune system without immune cells.

2. What Exactly Are Immune Cells?

Immune cells are white blood cells—and they’re your soldiers. “Our body uses different immune cell types to detect invaders, mount a defense response and kill,” says Dandekar. A couple of the big hitters are phagocytes and lymphocytes. The former surround and ingest invaders, while the latter memorize the names of all trespassers and then destroy them.

Say you get a cut or a burn. The infected or damaged cells send out a signal asking phagocytes (like macrophages and neutrophils) for aid. “They help with the early stages of a pathogen entry,” explains Dandekar. They’re the first soldiers out on the field, and they do their best to surround and capture the invader.

Chances are, you’re familiar with phagocytes. “When you have a classic bacterial infection—say a urinary tract infection, appendicitis or pneumonia—your doctor will do a blood test to check your white blood cell count, specifically looking for a high number of a type of neutrophils. If they’re there, a battle is brewing over a bacterial invader,” says Blum. If the enemy is too much for phagocytes to battle alone, they recruit lymphocytes for backup.

“Lymphocytes are the big guns, doing most of the fighting for you,” says Blum. All lymphocytes begin in the bone marrow. Some stay and become B lymphocytes. Some fall out to the thymus, situated behind your sternum, to become T lymphocytes. B cells are like the body’s military surveillance system.

You see, all cells essentially sport name tags that say “Hey! I’m an invader” or “Hey! I belong here!” These name tags are identification molecules called antigens, and specific immune cells, like B cells, are primed to recognize the invader name tags. They’re constantly searching for cells that don’t belong.

Once B cells zero in on an invader, they begin making proteins called immunoglobulins, better known as antibodies. These antibodies lock onto the invaders and sound an alarm throughout the body, alerting various soldiers on various bases to run to the front lines, guns blazing. Enter the muscle: T cells. “These are the main warriors flooding the frontlines, engaging in hand-to-hand combat,” says Blum.

They destroy the invaders that the antibodies have latched onto. One of the main pieces of artillery for T-cells are chemicals called cytokines. “Their purpose is to cause destruction and kill the invader,” says Blum.

3. How Do You Know Your Immune System Is Working?

Common signs that the battle is on—and that the immune system is winning—include pain, redness, swelling and heat. “They’re the classic signs of inflammation,” says Blum. “It shows that your injury or infection has been surrounded and knocked back.” Anyone who’s had a fever or a cut or suffered through the common cold is familiar with these ramifications. “Sneezing, congestion, fever, sore throat: all of these symptoms are due to the inflammatory response triggered by the immune system,” says Cassel. “The early sign of an infected wound—pus—is actually the debris of rapidly responding immune cells that are dying after gobbling up as many bacteria as they can.”

And swelling? In order to get all your immune cells onto the battlefield, your body sends in more blood and fluid to the area. (It’s their ride, after all.) All of the above is called innate immunity, and you’re born with it. “This type of immunity doesn’t change. Instead, the same invader triggers the same response each time,” Cassel explains.

Once the job is done and the battle is won, most of the white blood cells that have been called to action self-destruct and are stealthily absorbed into the body. “But some of those B and T cells linger and become memory cells, and they persist for years and years,” says Cassel. These cells remain at the ready to swiftly recognize and attack any returning invaders.

In fact, while it can take up to two weeks for your body to clear a virus the first go around, experts say the job can be done in roughly five days the next time the virus arrives, thanks in part to memory cells. “This is called adaptive immunity,” says Cassel, noting that this type of immunity develops throughout our lives via exposure to viruses and other sicknesses.

4. Where Do Vaccines Factor In?

If adaptive immunity sounds a lot like how vaccinations work, that’s because it is. With vaccines, your body doesn’t need to wait around to contract chickenpox to begin building protective memory cells and antibodies. Instead, “a vaccine introduces the invader’s name tag, spurring your immune cells to scout it out—take the information back to the lymphocytes to get the troops mobilized,” says Blum.

The difference between naturally contracting a virus and being exposed through a vaccine is that the vaccine version is either dead, super weak or just a fragment of the germ. “It’s not enough to make you sick, but it’s enough to stimulate your immune system to produce antibodies,” says Blum.

“Plus, the vaccine only stays in the body for so long. By the time antibodies are made, the vaccine has left your body, so all that you have left circulating are all these antibodies, waiting to find the enemy.” While vaccines are fantastic at imprinting anti-body-making memories in the body, memories can start to fade with age, and certain viruses can mutate. “That’s why some vaccines will require booster shots,” says Blum.

Unfortunately, we don’t have vaccines for every illness that strikes. “In general, the illnesses that respond to vaccines are ones with the longest memories,” says Cassel. Let’s rewind to before the chickenpox vaccine.

Back then, people would get chickenpox only one time, usually during childhood. That’s because the body’s cells held the chickenpox memory for a very long time, ensuring that protective antibodies lingered. “Other infections, like malaria or hepatitis C, don’t have that kind of memory and don’t result in that same type of immunity,” says Cassel. For example, while memory B cells do, in fact, increase after acute malaria, after six months, the plummet begins, according to a study in EMBO Reports. “This makes it much more challenging to design effective vaccines,” says Cassel.

Vaccines also factor into what’s called passive immunity. This is when a newborn baby borrows temporary immunity from her mother thanks to antibodies being passed to her before birth and via breast milk after birth. That’s why the Centers for Disease Control and Prevention recommends that women get a flu shot and the Tdap vaccine during pregnancy.

5. Does the Immune System Get Weaker with Age?

In short, yes. According to a 2020 report in the magazine New Scientist, the immune system’s gradual decline begins to affect our health at about age 60. “In our youth, we’ve got an abundance of naive immune cells that are primed to get educated about attacking pathogens,” says Dandekar.

“At the same time, our ability to replace and renew our immune cells is high.” But as we get older, our immune system’s capacity to generate a fast and robust immune response slows. For example, while older neutrophils can still detect invaders, they’re not as good at hunting them down. It’s like their GPS starts to malfunction.

“That means infections may take longer to resolve, and your immune responses to vaccines can be at a lower level,” says Dandekar. (This is why the CDC recommends that all folks 65 and older receive a high-dose influenza vaccine that contains four times the antigens of the standard vax.) At the same time, “the immune system’s vision can blur,” says Dandekar, and this can make it more difficult to distinguish friendly cells from invader cells, increasing the odds of things like autoimmune disease and cancer.

6. How Can I Improve My Immune System?

First, let’s toss the notion that you can significantly “boost” your immune system. You don’t want to turbocharge it to go into overdrive. “Instead, you want to keep it in balance,” says Cassel, noting that balance comes when the soldiers are well fed and the battlefields are clear. To nourish the troops, antioxidants are key.

“Our bodies are designed to use antioxidants from our diet to cancel out the stress involved in the battle,” says Lanford. Plus, they’re needed to build the strength to wage war in the first place. And when we’re lacking, it shows. For example, a 2019 study in the Journal of the American Medical Directors Association found that low antioxidant levels in people 50 and older delayed their ability to bounce back after an infection or surgery.

And our collective antioxidant shortage accelerates the development of conditions such as cancer and cardiovascular disease, notes a report in the journal Food and Bioproducts Processing.

Along with antioxidants, a gut-healthy diet rich in fiber, plants and fermented foods is key for the health and function of your immune cells. Remember: the majority of the body’s immune cells are in the gut. “You’ve got trillions of bacteria lining your gut, communicating with your lymphoid system at all times,” says Blum. “Keeping all of that nourished helps to ensure that your lymphocytes develop properly.”

At the end of the day, eating a range of nutrients as part of a varied yet plant-heavy diet is best. And while downing extra vitamin C or popping a zinc lozenge at the first sniffle may help your immune soldiers battle, know that “it’s hard to make up for a nutrition or antioxidant gap once you’re in the throes of illness,” says Blum. “It’s a better strategy to always keep your nutrition baseline high.”

It’s not just nutrients that influence how primed our soldiers are for the battle. Physical activity, weight, sleep, stress and smoking habits have great sway over the operation too. For example, a report in the journal Brain, Behaviour, and Immunity found that the neutrophils in older adults who clocked 10,000 steps a day were just as healthy as the neutrophils in young adults.

Plus, active muscles are antiinflammatory and stimulate macrophages, another type of immune cell. On the flip side, obesity is associated with a decline in B cell function and poor vaccine response, even in people who are young. “We also know that lack of sleep can impair the production of cytokines,” says Cassel. (Those are your infection-fighting T cells’ best weapons.) “It also triggers the release of stress hormones that further dampen the immune response.”

7. Do you know what else triggers stress hormones?

Stress itself. Cortisol levels that are too high can hinder wound healing, lower antibody response to vaccines and increase susceptibility to infections and a variety of diseases, according to a 2016 report. Smoking, it seems, has a similar impact, leaving folks more vulnerable to illnesses like cancer, heart disease, diabetes, rheumatoid arthritis and more. “We’re still learning about the mechanism of it all, but we do know that nicotine interferes with the ability of certain immune cells to gobble up debris and bacteria,” says Cassel.

The good news? It’s all modifiable. “You can positively impact your immune system at any age, even if you never did those things before,” says Blum. The immune system is complicated, but helping it to run smoothly is remarkably simple. “Everything you’d do for general overall health will also help improve the health and function of your immune system,” Blum says.